What is chromaticity?
The colour of a light written as two numbers, x and y, independent of how bright the light is.
Chromaticity describes the colour of a light, separate from how bright it is. It is given as two numbers, x and y, which place the light as a single point on the CIE chromaticity chart.
Two numbers are enough because the three CIE values add up to one. The chart is bounded by a horseshoe-shaped curve that encloses every colour the eye can see. For AGL, the standard marks a region on this chart for each signal colour: red, yellow, green, blue and white. A light is the right colour only if its point lies inside the region for that colour. Brightness is a separate property, carried by candela, so a light can have enough intensity and still be the wrong colour.
Frequently Asked Questions
Q1What is chromaticity?
The colour quality of a light, given as two numbers, x and y, on the CIE chromaticity chart. It says what colour the light is, regardless of how bright it is.
Q2Why are two numbers enough?
The three CIE values add up to one, so once x and y are known the third is fixed. That reduces colour, apart from brightness, to a point on a flat chart.
Q3Does chromaticity change with brightness?
In principle no: dimming a light by scaling it leaves x and y unchanged, and brightness is carried separately by candela. In practice a dimmed filament lamp shifts toward yellow, and an LED can shift colour a little with current or temperature, which is why colour is checked.
Q4What is the horseshoe shape on the chart?
The boundary of all colours the eye can see. Its curved edge holds the pure spectral colours and the straight edge closes it. Every real light sits inside it.
Q5Why does each AGL colour have its own region?
A pilot must tell red from yellow from green quickly and without doubt. The standard sets a region on the chart for each aeronautical ground light colour, and a light must fall inside the region for its colour.
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